Malaria has long presented the most serious of global public health problems among the infectious diseases. Attempts to control the mosquito vector and use of antimalarials notwithstanding, there are yet some one million fatalities annually from the disease. Not clearly evident in the mortality statistics are the vast range of problems related to the tens of millions of cases suffering from morbidity derived from malaria infections. In part, the extent of the economic and public health problems derived from malaria has been related to difficulties in the chemotherapy of that protozoan disease. The narrow spectrum of action of antimalarial drugs in relation to the life cycle of the parasite has been clearly apparent, for example. Development of drug resistance by Plasmodia has further complicated a situation which of necessity includes use of compounds which may be poorly tolerated by many individuals.
Treatment of acute malaria is an urgent medical problem, and may constitute a grave emergency lest there be fatal consequences. Acute malaria is a result of the presence of Plasmodia in the bloodstream, and those parasites must be eradicated to give clinical cure. Malaria resulting from infection with Plasmodium falciparum frequently leads to a severe pathophysiologic cascade and death may occur soon after onset of symptoms. Elimination of blood forms of the parasite ordinarily clears the body of Plasmodium falciparum (clinical cure of malaria). Infection with Plasmodium vivax (and also the rarer parasites, Plasmodium malariae and Plasmodium ovale, to variable extent) gives rise to a considerable reservoir of tissue forms (exoerythrocytic stage) of the parasite which are able to cause relapses of malaria through intermittent reinvasion of the blood, as, following treatment of the original attack. Thus, alleviation of acute vivax malaria with a (clinical) curative drug does not perforce achieve elimination of all parasites from the body and produce a radical cure. Only when all of the organisms have been cleared from blood and formed tissues can there by freedom from possible relapse of the malaria. Mixed infections in people, as with Plasmodium falciparum and Plasmodium vivax, require treatment including both clinically curative and radical curative drugs to afford actual eradication of the malaria.
In clinical practice, the management of acute malaria may well follow differing patterns, depending upon the parasite and the severity of the infection. When acquired in regions where falciparum malaria is responsive to chloroquine, that drug or other 4-aminoquinoline may be used in oral treatment of the condition. If the falciparum parasites may be resistant to chloroquine, or if obviously severe infection is presented, treatment may well be by an intravenous infusion of quinine followed by oral administration of mefloquine or of pyrimethaminesulfadoxine combination. Acute vivax malaria (or, malariae malaria, or ovale malaria) ordinarily responds well to chloroquine treatment. Prevention of relapses due to persistent tissue forms of Plasmodium vivax (and, Plasmodium malariae, or Plasmodium ovale) requires use of a radically curative agent.
Primaquine is one drug that serves as a clinically effective radically curative antimalarial drug. It clears the tissues of malaria parasites (i.e., tissue schizonticide) and also kills sexually differentiated forms (gametocytes) in the blood at clinically usable doses. On the other hand, primaquine is relatively ineffective against the blood schizonts (i.e., little of blood schizonticidal action) which evoke clinical symptoms of malaria. The toxicity of primaquine precludes administration of doses which would be sufficient to treat overt cases of malaria.
It has become apparent that palliative effects of chloroquine are achieved with safe doses of the drug, whereas primaquine may give evidence of toxic effects even at therapeutic doses [cf. World Health Organization report WHO/MAL/79.905(1979), H. Weniger]. Thus, chloroquine must be administered to scavenge schizonts from the blood while primaquine destroys tissue forms. It appears that the problem with use of primaquine for both effects is two-fold, viz., inadequate blood schizonticidal activity and undue toxicity.
In the course of the U.S. program on antimalarials research during 1941-1945, the serious attempts made to improve the profile of pamaquine led to primaquine. Some investigations of 8-aminoquinolines were continued toward broadening the effectiveness and decreasing the toxicity of primaquine. One approach was to alter the basic side-chain. A representative was 6-methoxy-8-(5-propylaminopentylamino)quinoline phosphate [U.S. Pat. No. 3,096,334 (2 July 1963) E. A. Steck; J. Org. Chem., 24, 700(1959) E. A. Steck with L. T. Fletcher], which had sufficiently good profile as a blood- and tissue-schizonticide in the laboratory [Antibiotics Chemother., 12, 103(1962) D. A. Berberian, et al.] that it underwent field trails [e.g., Bull. W. H. O., 32, 591 (1965) R. D. Powell]. It became apparent that such structural modification achieved no appreciable improvement in overall worth of primaquine. Other approaches toward enhancing effectiveness of primaquine included synthesis of 4-methyl primaquine [J. Am. Chem. Soc., 77, 4816(1955), R. C. Elderfield, et al.]. Although previous test systems failed to identify it as superior to primaquine, recent improvements in the evaluation of antimalarials [cf., Am. J. Trop. Med. Hyg., 24, 174(1975), K. E. Kinnamon and W. E. Rothe; ibid., 27, 718(1978), L. H. Schmidt; ibid., 28, 937(1979), D. S. Rane and K. E. Kinnamon] so indicated. Note may also be taken of the synthesis of 5-substituted 8-aminoquinoline derivatives during the World War II program of antimalarial research in the U.S.A.: cf. The Chemotherapy of Protozoan Diseases, by E. A. Steck (published 1972 by Walter Reed Army Institute of Research, Washington, D.C.)--Volume 3, pages 23.160 to 23.162.